Current research in nanophase materials has been inspired by the discovery that when the scale of the microstructure is less than about 5 nanometers new properties emerge, which are completely unexpected by simple extrapolation of properties found for larger scale microstructures. Another incentive as been the realization that significant improvements in properties can be achieved simply by diminishing the scale of the microstructure, while preserving chemical homogeneity and microstructural uniformity. Thus, the focus of attention has been on the synthesis and processing of materials with ultrafine microstructures in the range 1-100 nanometers. Such structures cannot be produced by conventional casting and mechanical working methods. To overcome these limitations other approaches have been developed, such as rapid solidification, sol gel synthesis, and cryomilling.
According to the Hall-Petch relationship a reduction in the grain size of a material leads to an increase in the yield strength. The quantitative relationship states that the yield strength is inversely proportional to the square root of the grain size. For brittle ceramic materials the fracture strength increases with decreasing grain size in a similar manner. Furthermore, multiphase microcrystalline materials frequently exhibit superplastic properties when deformed in appropriate temperature/strain rate regimes.
When the grain size of a material approaches 2 nanometers, there are just as many atoms encompassed within the grain boundaries as within the grains themselves. Thus, the properties of the materials are strongly influenced by the properties of the grain boundaries. Recent studies have shown striking modifications in the properties of metals when they are in the nanocrystalline state, as shown in Table 1.
TABLE 1 ______________________________________ Properties of nanophase metals compared with their crystal counterparts. The percentages in parentheses represents changes from the reference crystal value. After Birringer et al.* Property Nanocrystal Units Material Crystal ______________________________________ Thermal Expansion 10.sup.-6 K.sup.-1 Cu 17 31 (+80%) Density g-cm.sup.-3 Fe 7.9 6 (-25%) Saturation emu-g.sup.-1 Fe 222 130 (-40%) Magnetization @ 4K Susceptibility 10.sup.-6 Sb -1 20 (N/A) emu-Oe.sup.-1 g.sup.-1 Fracture Stress kP-mm.sup.-2 Fe (1.8%C) 50 600 (+1000%) Superconducting Tc K Al 1.2 3.2 (+160%) ______________________________________ *Barringer et al., Phys. Lett., A102(1984), 365
In a multiphase material, similar effects can occur but the situation is complicated by the tendency for unlike phases of small dimensions to exhibit crystallographic coherency. One manifestation of this is the appearance of a supermodulus effect in multilayer thin films, where coherency is maintained over large distances in the periodic structure. Physical properties are also changed, for example, quantum well effects in thin film semiconductors and opto-electronic materials.
Chemically synthesized nanophase metal-ceramic composites have many potential applications. These include tool materials, such as cobalt-bonded tungsten carbide (WC--Co), dispersion strengthened materials, such as alumina-strengthened copper (Al.sub.2 O.sub.3 --u), conductor-bonded high Tc superconductors, such as silver-bonded yttrium/barium/copper oxide (YBa.sub.2 Cu.sub.3 O.sub.7-x --Ag) and whisker reinforced composites, such as silicon carbide reinforced alumina (SiC--Al.sub.2 O.sub.3).
Nanophase composites can be prepared from metals or oxides mixed at the nanoscale level. Reduction of mixed oxides would give an intimate dispersion of the more stable oxide in a pure metal. Selective oxidation of a mixed metal would give the same result. Mixtures of metals can be converted to a number of composite systems by heat treatment in appropriate gas mixtures, e.g., CO--CO.sub.2 for metal-metal carbide cermets, or NH.sub.3 --H.sub.2 for metal-metal nitride cermets.
Although this invention is not limited to a single composite system, for the purposes of describing its essential features we will use the thermochemical processing of nanophase WC-Co composite powders as the primary example. Other notable hardmetal composites to which the present invention would be applicable are listed in Table 2.
TABLE 2 __________________________________________________________________________ Examples of Cemented Carbide Materials** WC Based WC-Free Years Cemented Carbides Years Cemented __________________________________________________________________________ Carbide 1922-25 WC-Co* 1927 Graphite Free WC + Co 1928-29 WC + Stellite Binders 1929-31 TiC + MoC + Ni,Cr,Mo 1931 WC + TiC + Co* 1930-31 Tac + Ni,Co WC + TaC(VC,NbC) + Co 1931 TiC + Cr,Mo,W,Ni,Co* 1932 WC + TiC + (TaNb)C + Co 1931 TiC + TaC + Co 1938 WC + Cr.sub.3 C.sub.2 + Co 1938 TiC + VC + Ni,Fe 1944 TiC,NbC + Ni,Co 1948-50 TiC (Mo2C, TaC) + Ni, Co, Cr 1949 TiC + VC + NbC + Mo.sub.2 C + Ni 1951 WC + Ni 1952-81 TiC + Heat Treatable Binder 1956 WC + TiC + Ta(Nb)C + Cr.sub.3 C.sub.2 + Co 1959 WC + TiC + HfC + Co 1965 HIP 1965-70 TiC + Mo2C + Ni,Mo 1966 Submicron WC-Co 1968-69 WC + TiC + (TaNb) 1968-70 TiMoC + Ni,Mo,Cr C + HfC + Co 1969-80 Coated Carbides Tools* (C-5 Substrates)* 1975 Cast Carbide 1976- Coated Carbide Tools 1976 (W,Mo)C + Co Present on Tailored Substrates __________________________________________________________________________ *Critical Development **References: 1. R. Kieffer and F. Benesovsky: `Hartmetalle`; 1965, Wein, New Yor, SpringerVerlag. 2. K.J.A. Brookes: `World directory and handbook of hardmetals`, 2 ed.; 1979, London, Engineer's Digest. 3. R. Kieffer ad P. Ettmayer: Chem. Ing. Tech. 1974, 46.843 4. H.E. Exner: `International Metals Reviews
The traditional method of making WC--Co cemented carbides is by crushing, grinding, blending and consolidation of the constituent powders. Thus, the microstructural scale can be no smaller than the size of the milled powders, typically 1-10 microns in diameter. Using a new chemical synthesis method, pioneered at Exxon and Rutgers as disclosed in pending U.S. application Ser. No. 053,267, see also R. S. Polizzotti, L. E. McCandlish, Solid State Ionics, 32/33, 795 (1989), the constituent metallic elements are premixed at the molecular level, thereby permitting control of chemical and microstructural uniformity on the submicron scale. Table 3 illustrates the essential differences between the traditional powder metallurgy method and the new chemical synthesis method for making WC--Co powder. As shown, the chemical synthesis method provides a more direct route for making the composite powder.